In the field of headphone design there is often a need to either create specific custom frequency responses, or to flatten the existing response of the headphone speaker driver. The usual techniques to achieve this are either to carefully design the transducer (headphone driver) to modify or flatten out the frequency response; or to add digital signal processor (DSP) technology to allow parametric EQ to be performed. Both of these techniques have significant disadvantages. For example, using only the design parameters available within the transducer as is usually done has several drawbacks, such as inability to adjust for variability in mass production. Moreover, there are limits on the frequency response changes it is possible to make without increasing the transducer cost to an unacceptable level. For the case of DSP equalization, there must be a power source available. This may not be an issue with headphones that already include a power source, such as headphones that include Bluetooth streaming functions. But for basic passive headphones, adding the cost of DSP processing and a power source is generally also unacceptable.